From Halopedia, the Halo wiki
Modding is the act of altering aspects of a game by manipulating its program code and other resources. A number of things qualify as mods, ranging from palette swaps to the creation of entirely new maps. Some mods involve the alteration of game resources, while others run independently of a game and edit its working memory. Because modding is often used for cheating, it is not allowed on Xbox Live and the use of a modded Xbox on Xbox Live will result in a terminated account.
Modding in the Halo series can be done across all games on PC and Xbox 360 platforms, with the caveat of not being connected to Xbox Live. Halo Custom Edition, a port of Halo: Combat Evolved for PC, is specifically designed to allow for user mod support and custom content, while Halo: The Master Chief Collection allows players to disable the game's anticheat software at launch to enable modding and disable Xbox Live functions such as multiplayer matchmaking.
Mod compatibility by game
Mod support for Halo titles depends depending on the game and platform in question - notably, Xbox One games have very little modding support.
Assembly - Releases A tool for Real Time Edits on Halo 2, Halo 3, Halo 3: ODST and Halo: Reach. This program is easy to learn and allows for modding of the preferred choice of Halo game
Assembly allows the users choice of Halo: 2, 3 or Reach to be open while editing in-game Tags Real Time.
Ascension - Tag editing tool for Halo Reach (similar to Assembly)
Halo: Combat Evolved
Halo Custom Edition, an expansion to Halo PC, allows users to load and play user-made maps. Because all of the game's assets are stored as "tags" in map files, virtually everything that isn't part of the core game engine can be altered by a mapmaker. This, as one might expect, includes core map geometry, AI scripts, map scripts, textures, weather and skybox effects, sounds, weapons, and vehicles. This system, combined with the availability of programs that extract tags from official campaign maps, makes it possible for mapmakers to create custom single-player campaigns, complete with cutscenes.
"Hacks" are technically another form of modding. Hacks generally take the form of third-party programs that a user activates after starting the game. They operate by reading and editing the game's working memory. Hacks usable by clients include aimbots, sightjacking programs, third-person point-of-view and flycam utilities, heads-up display-disabling programs, texture modifications, name hacks, and draw distance changers. Hosts can use a wider variety of hacks without causing desynchronizations; these include dedicated server administration tools, weapon mods, hacks that can change individual players' properties on-the-fly, and even a tool that recreates Halo 3's Infection mode.
Mods and hacks can be used to gain access to retail-only gametypes in the trial version of Halo PC. Programs exist that can convert retail-only maps into demo-compatible maps.
Halo 2 Xbox
It is possible to modify maps that are installed to the hard drive, such as Containment (Level) and Terminal (map). Because the Halo 2 engine still uses the "tag" system from Halo: Combat Evolved, it is therefore possible to edit any resource or asset in the map. However, mods will not synchronize for all players unless the person using the modded map is host. (So if a client modded Containment to make SMGs fire Scorpion rounds, and then they joined a game hosted by a non-modder, only the client with the mod would see Scorpion rounds come out of their SMG.)
Halo 2 Vista
Halo 2 Vista has software that allows RTE (Real Time Edits) to the campaign levels. This tool is Assembly "Not Related to the Level Assembly" This tool will work on Halo 2 Vista's Campaign along with a few other Halo games! The software does not have protection for Multiplayer maps, but instead hides all Tags from normal multiplayer .map files and puts them in a single file. RTE's Allow the Game (Halo 2 for Windows Vista) to be open while the mods are injected/poked into the game by editing values! The mods are removed when you leave the Mission/Multiplayer! Although you can make SPPF patches with Assembly then later you can use an SPPF Patcher to apply them to the default .MAP files! MAKE BACKUPS OF YOUR .MAP FILES, THEY COULD CORRUPT!
Cheating With Assembly/Tag Editor's
With the release of Halo 2 online, cheating in Halo 2: Project Cartographer has been banned. An advanced system has been enforced to make sure you remain banned if you cheat online!
Console-based mods became very widely-available in Halo 3, thanks to the development of the File Share and other ways of transferring user-created content. Though modding on the scale of Halo Custom Edition cannot be done without a devkit or a modded console, user-created Files can be altered using simple utilities to produce unusual effects.
Bungie does not allow the uploading of modded map or game variants to one's File Share, nor do they allow the uploading of modded Films, Film Clips, or screenshots. However, they do allow the uploading of un-modded Screenshots and Films showcasing modded map and game variants. After viewing a Film or Film Clip, the map and game variants from that Film are stored in a player's Recent Maps and Recent Gametypes; this allows players to share modded maps and gametypes by simply sharing un-modded Films of those gametypes on those maps.
Jtag/RGH/Devkit mods are essentially limitless. Any object in any map can be altered to do anything. Examples include:
Map variants, for example, can be created in Forge and then modded. Forgeable objects can be replaced with any non-terrain objects in a map. Through this method, a variety of development leftovers can be added to maps. These include partially-Forgeable objects from the Halo 3 Epsilon build (Deployable Lookout Towers and Shades), unused game objects (like Rocket Launcher ammo cases and Needler ammo pickups on Blackout), and unused map-specific objects (like a hologram of the Ark on Guardian). Some vehicles cannot be Forged onto a map, but can be modded onto the map; examples include Hornets and Banshees on Narrows, and Warthogs on The Pit. Warthog turrets can be placed independently of a Warthog. Map-specific objects (like Sandtrap's Elephants and Guardian's birds) can be created and moved using modding. Objective objects, such as bombs, skulls and flags, can be modded onto a map, in which case they are available for use in all gametypes. Inactive bipeds (Spartans, Elites, and Monitors) can also be placed on maps. Even large pieces of map scenery, such as the UNSC Aegis Fate on Sandtrap, can be modded, though such mods are only visible to the host.
Gametypes also have the .blf (Blam File Header/Blam Library Format) header, which stores information such as the Variant Name, Variant Author, and Variant Description. These fields may be edited to hold any value, including censored terms and control characters, and displayed in the menu. Control characters allow symbols and icons to be inserted into the map's description. An XML-like syntax allows colored text, the current date and time (updated in real-time), and the name of a viewer's gamertag to be inserted into the description. It can become difficult for any information about the file to be verified locally, including the original author.
Screenshots and Films
Players in Halo: Reach have been able to use offline mods to unlock armor pieces prematurely, "fly" UNSC Frigates in Long Night of Solace (by "attaching" the Frigates to Sabres), "fly" the Pillar of Autumn on its titular level (using the same method, but with a Banshee), pilot Seraphs and other normally-unusable vehicles, use Jackal shields and other normally-unusable items, and use a Bungie-exclusive font glyph (the Bungie logo) as a Service Tag.
Devkit mods also revealed hidden functionality in the Beta. One could join and host Local and System Link games, and break into the Custom Game lobby via the Theater lobby. (Gametype settings were entirely non-functional, though whether this is due to the Beta or due to the devkit mods is unclear.) As in Halo 3, weapons could be altered to fire different projectiles or spawn new objects, like vehicles.
Bungie takes action
This has become such a widespread problem in the Matchmaking system that Bungie was forced to resolve the problem by releasing auto-updates which ban hackers from the system and terminate their accounts. Bungie has banned thousands of players and is working to clean up the rest, earning in the process the moniker 'Banhammer.' Several sites offer a list of cheaters and hackers.
Although there used to be several ways to mod, Xbox Live and Bungie have collaborated to rectify the programming mistakes and prevent the game from being modded as much. They are also reinforcing much stricter punishments for cheaters, such as instant bans and longer suspensions.
Halo 5: Forge
The Exuberant mod tool by YouTuber GameCheat13 allows players to:
You cannot be banned for using Exuberant unless you are using options from the "Local Only" tab. However, enabling AI in forge online will result in a ban.
Because Halo 5: Forge does not have Arena or Warzone, player progress on Xbox One will not be harmed.
Halo: The Master Chief Collection
While modding is not officially supported by Halo: The Master Chief Collection, the game does allow for it by enabling users to disable the game's anticheat software prior to launch. While anticheat is disabled, players cannot enter matchmaking and progression progress is not saved to the Halo Waypoint servers. However, players will not be punished for any mods run while anticheat is disabled. Some mods such as menu background replacers can be run with anticheat enabled without penalty.
To mod The Master Chief Collection, there are several programs available. Due to the games in the collection being those available for the Xbox 360, Assembly is compatible with MCC files and tags. A full list of useful software is below;
A large selection of user-created mods can be found at The Master Chief Collection on Nexus Mods.